1. Field
The following description relates to a technology for detecting signal level on an electronic circuit, and, more particularly, to a technology for detecting signal level quickly and a technology for actively controlling gain of a Trans Impedance Amplifier (TIA) by associating a signal level detector with the TIA.
2. Description of the Related Art
A technology of detecting a signal level is essential for detecting or restoring data without an error in accordance with a change in a level of a wide-range input signal. Thus, this kind of technology is applied widely to a system for receiving a wide dynamic range signal, such as a wired/wireless communication system, a measurement equipment, a bio healthcare and medical device, and a disk drive.
The technology of detecting a signal level provides various key functions, such as detecting or restoring data, by applying an Automatic Gain Control (AGC) with a peak detector to automatically control gain corresponding to a change in a level of an input signal. In particular, a point-to-multi-point (P2MP) Passive Optical Network (PON) is configured in an optical communication system, optical packet signals from multiple subscribers have different path loss values and thus consist of burst signals with different signal levels. In this case, in order to restore a burst packet receipt signal without data distortion, the burst packet receipt signal whose level is constantly changed at predetermined time intervals, a receiver in a base station essentially needs to have a Burst-Mode Trans Impedance Amplifier (BM-TIA).
In general, in order to detect a peak value of a signal, a rectifier as shown in FIG. 1 is used. FIG. 1 is a diagram illustrating a configuration of a general rectifier having a passive device.
Referring to FIG. 1, a diode D 10 is configured in a manner where forward current flows from Vpeak node toward Vs node and reverse current from Vpeak node toward Vs node are blocked. Using a characteristic of the diode D 10 that lets current flow in one way, constant voltage is retained through a capacitor C1 12 and a resistance 14 for a predetermined time period so as to detect a peak value of a signal.
FIG. 2 is a diagram illustrating a peak detector used in a general integrated circuit for high-speed data processing.
Referring to FIG. 2, a peak detector includes an amplifier 20 that consists of a current source and transistors M1, M2, M3, and M4. The amplifier 20 has a positive node and a negative node, wherein an input signal is input (In) to the positive input node and the negative node has a negative feedback loop circuit configured therein. In the negative feedback loop circuit, output of the amplifier 20 is provided as an in-phase value that is source followed through a rectifier consisting of a diode D1 21 and a capacitor Chold 22. To reduce an error between two inputs, the amplifier 20 constantly compares the two inputs and amplifies a differential therebetween. As a result, through a buffered rectifier output, signal peak values are obtained.
The structures described with reference to FIGS. 1 and 2 are stable but may cause various problems in an optical communication system, such as a Next Generation Passive Optical Network 2 (NG-PON2), which needs to extract a signal level as fast as possible within a few ns˜tens of ns.
The first problem is caused by a frequency characteristic of a diode used for rectifying a signal. A rectifier diode seems to usually act as a capacitor component in an optical communication clock frequency at a multi-Gbps. It means that the rectifier diode is unable to perform a forward rectifying function at a low frequency. Even a schottky barrier, which is used for high-frequency operations, is unable to normally perform a rectifying function in an optical communication clock frequency, so that it takes a great deal of time to detect a peak value of a signal.
FIG. 3 is a simulation result graph regarding comparison in terms of a frequency characteristic and a transient characteristic in the case where a rectifier consists of a general diode used for an integrated circuit and a schottky barrier diode.
Referring to FIG. 3, both of the general diode and the schottky barrier diode are at −3 dB, which is a low frequency (within 1 MHz), showing characteristics of a Low Pass Filter (LSP); however, at a much lower frequency, the both reach to zero where gain attenuation does not occur. It means that if a multi-Gbps clock signal is input, the signal is a bit attenuated but able to pass.
Regarding the transient characteristic, the signal is able to be output in a manner described above. It may be found that, in the case where an output value is a few ns˜tens of ns, both of a general diode and a schottky barrier diode are not able to reach a peak value of a signal and a great deal of time is required to detect the peak value. Thus, it is hard to apply a rectifier using a diode to a multi-Gbps optical communication system.
The second problem is related with the fact that burst packet signals with different levels are input to a P2MP optical communication system every hour. To address the situation, a preamble signal exists for a predetermined time period before actual data is put. The preamble signal is a high-speed clock switching between 0 and 1, and gain may be controlled by detecting a signal level for the predetermined time period. Using the peak detector configured to include an amplifier and a rectifier, as shown in FIG. 2, the maximum level of a detected signal may be only half an average level of an input signal. It is because an average between 0 and 1 is obtained due to charging and discharging of a parasite capacitor component. As a result, a level smaller than a peak value of the signal is detected, so that resolution may be debased. In addition, a time constant of an internal rectifier is great, and thus, a great deal of time is required.
In addition, a peak detector usually receives output from a TIA disposed on the front end thereof for a short response time. Thus, a signal level is not high. Considering that an input signal current is at between tens of uA˜hundreds of uA, signal amplitude that the peak detector has to detect needs to be very small within a range of hundreds of uV˜tens of mV. Under the environment where electric noise of the digital system exists, an extraction value obtained by detecting a signal of a small output level is only half a level of the aforementioned preamble signal, so that a discriminatory capability of the peak detector may become further deteriorated. Thus, it is hard to apply the conventional structure to the multi-Gbps optical communication system.
The third problem is that the aforementioned configuration is hard to cope with change of temperature or a processing parameter under the environment where resolution is debased and digital noise exists. Thus, there is need for an alternative for stably detecting a peak value although temperature or a processing parameter is changed. Furthermore, an integrated diode occupies more space, and a process for a high-frequency diode, such as a schottky barrier diode, needs to be supported for high-speed operations.